Recent Association of American Railroads (AAR) regulations concerning railway tank cars require that projections extending below the tank bottom in excess of one (1) inch must be protected by a skid which adds weight to the car and expense to install.
Many railway tank cars are operated by a rod extending from a lading closure plug in the bottom of the tank upwardly through the tank to a conventional operator located at the top of the tank. However, it would be desirable in some situations to be able to operate the lading closure plug from the bottom of the tank, in addition to normal operation from the top of the tank.
Furthermore, it sometimes is desirable to be able to remove or move the lading closure plug between open and closed positions from the bottom of the tank, without the operator's having to go inside the tank.